Flexible hoses having core tubes made of elastomeric or flexible plastic materials require reinforcement by one or more layers of stainless steel wire, nylon, fiberglass or the like when the hoses are to be used for conveying fluids under high pressure. Each layer may comprise first and second sets of oppositely helically wound strands that may be either interwoven to form a braid or knit or wherein the second set is wound over the first set to form what is sometimes referred to as a spiral wrap. In hydraulic service the pressure within the hose may be over 1000 psi. For small diameter hoses, such as 0.250 inch ID, one layer of reinforcement may be sufficient to give the hose a burst strength of more than 10,000 psi, depending upon the particular reinforcement material used and the amount of coverage provided by the reinforcement for the core tube. The strands may be of multiple filament or mono filament form.
More than one reinforcing layer frequently is required to provide the necessary burst strength, particularly for larger hoses, such as 0.500 inch ID and over. When two or more layers of reinforcement are used, it is preferable from the standpoint of ease of manufacture to make the two layers identical with respect to material, denier, percent coverage, angle of lay and method of application to the core tube, that is, whether spirally wrapped, braided or knitted. However, in such cases the layers do not share the load imposed by fluid pressure equally between them. The reason for this is that when fluid pressure is applied to the inside of the core tube, the core tube expands in diameter slightly to apply load to the first layer of reinforcement. This first layer then expands in diameter to apply load to the second layer, but each succeeding layer will not expand as much as the preceding layer.
Expansion of the layers comes about because of elongation of the filaments due to tensile stress therein and may also be due in part to the angle of lay of the filament on the core tube when such angle is substantially the same or greater than the neutral angle. Because the innermost layers expand more in diameter than the next succeeding outer layer the filaments of the inner layers are under greater tensile stress than the next outer layer when the materials and angle of lay are the same and are bearing a proportionately greater share of the load imposed by the fluid under pressure. Therefore such inner layers are under greater tensile loading than the next outer layer. As fluid pressure increases, the innermost layer will reach its break point while the next outermost layer is still loaded so as to be considerably below its break point. Thus, full advantage is not taken of the strength of succeeding layers and a greater amount of reinforcing material must be utilized in the innermost layer to achieve a given burst pressure for the hose than would be the case if all reinforcement layers were equally stressed under full load from the fluid pressure.
Because the actual load carried by the second layer of reinforcement in a hose having two such layers is difficult to calculate, it is common industry practice to rely on no more that a 50% increase in burst strength of a hose when adding a second layer of reinforcement identical to the first, whereas if the load could be shared equally by the two layers the increase in burst pressure would be substantially 100%.
A second problem that arises in connection with reinforced hoses involves hydraulic shock pressures. When a hose is installed in the high pressure portion of a hydraulic system, increases of pressure will cause hydraulic shock unless the increae in pressure is cushioned so that pressure rises will not be relatively rapid. When hoses are reinforced with materials of relatively low modulus of elasticity, which results in low strength, the hose will expand in diameter, and hence in internal volume, more readily when fluid pressure rises than if the reinforcing material has a high modulus of elasticity and hence a high yield strength. If the increase in hose internal volume is too rapid the pressure increase will be relatively slow and may result in delayed response or mushiness of operation of the hydraulic system. On the other hand, if such volume increase is too slow the pressure increase will be relatively rapid and result in shock pressures that may be detrimental to the system.
U.S. Pat. No. 3,905,398 describes hose constructions having more than one layer of reinforcement and wherein the inner layer is of a material having a relatively high modulus of elasticity and a relatively low elongation at break while the material of the second layer has a lower modulus of elasticity and a higher elongation at break than the first layer. In such a hose, the strands or filaments of the first layer expand relatively little under high loading and consequently transfer very little of the load to the second layer. Also, there is relatively little give by the first layer of reinforcement when fluid pressure is applied so that high shock pressures may be transmitted with little dissipation.